As illustrated in FIG. 13, a holder 100 includes: a pair of ring-shaped side portions 101 opposed to each other; and pillar portions 102 that integrally couple the pair of ring-shaped side portions 101. As also illustrated in FIG. 14, pockets 103 are respectively formed between the large number of pillar portions 102, and a needle roller 90 is inserted into each pocket 103.
Each pillar portion 102 includes: a larger-diameter portion formed at both the ends thereof, the larger-diameter portion having the same diameter as that of the ring-shaped side portions 101; a smaller-diameter portion formed in the center thereof; and an inclined portion formed between the larger-diameter portion and the smaller-diameter portion of the pillar portion 102. Further, a flange portion 104 is formed by bending an end portion of each ring-shaped side portion 101 inward. Because the holder 100 has such a shape as described above, a cross sectional shape of the holder 100 is an M shape, and the holder 100 is referred to as an M-type holder.
FIG. 15A, FIG. 15B, and FIG. 15C illustrate a conventional method for producing the above-mentioned M-type holder 100. A working target article 200 to be formed into the holder 100 is set onto a lower punch 300. The diameter of an upper end portion 300a of the lower punch 300 is the same as the inner diameter of the smaller-diameter portion of the holder 100. Moreover, the diameter of a middle portion 300b of the lower punch 300 is the same as the inner diameter of the larger-diameter portion of the holder 100. Then, an inclined portion 300c corresponding to the inclined portion of the holder 100 is formed between the upper end portion 300a and the middle portion 300b. Moreover, the diameter of a base end portion 300d of the lower punch 300 is the same as the outer diameter of the larger-diameter portion of the holder 100, and the working target article 200 is supported by the base end portion 300d. Moreover, a die 301 is provided around the lower punch 300, and the periphery of the working target article 200 is restrained by the die 301. Further, an upper punch 302 is provided above the lower punch 300.
Here, as illustrated in FIG. 15B, if the upper punch 302 moves downward, an upper end portion of the working target article 200 is pressed downward to be gradually bent inward.
Then, as illustrated in FIG. 15C, if the upper punch 302 moves further downward, the flange portion 104 is formed in the upper end portion of the working target article 200.
As illustrated in an enlarged manner in FIG. 16A, a thinned portion 200a is formed in a part to be formed into the flange portion 104, of the working target article 200 used in the method for producing the M-type holder 100. The reason why the thinned portion 200a is formed as described above is to enhance the bending formability. Consequently, as illustrated in FIG. 16B, sagging (the phenomenon in which the radius of curvature of a bent portion becomes larger than that of a plate thickness) can be prevented from occurring in a bent portion (R portion) 100a, when the flange portion 104 is formed.
In the above-mentioned method, in the case where the amount of compression by the upper punch 302 is increased in order to further suppress sagging, the working target article 200 deforms because of insufficient restraint of the smaller-diameter portion of the working target article 200. To deal with such deformation, post-processing of removing a part indicated by diagonal lines in FIG. 16C is required, and this decreases the yield and productivity.
A producing method in which a split mold for restraining the inside of the working target article 200 is used is known as a producing method capable of suppressing the occurrence of such sagging (see Patent Literature 1). FIG. 17A, FIG. 17B, and FIG. 17C are explanatory views illustrating a producing method in which a split mold 304 is used. In this producing method, the working target article 200 is set into a periphery restraining mold 303 having the same inner diameter as the outer diameter of the larger-diameter portion of the working target article 200. An outer peripheral surface portion having the same diameters as the inner diameter of the larger-diameter portion and the inner diameter of the smaller-diameter portion of the working target article 200 is formed in each split portion 304a of the split mold 304. Moreover, the split mold 304 can move upward and downward, and the split portions 304a of the split mold 304 can move also in the radial direction as also illustrated in FIG. 18.
Moreover, a producing method in which bulge forming is used is also known as another producing method of this type (see Patent Literature 2 and Patent Literature 3).